The objective of our research is the identification, purification and characterization of the components of the rough endoplasmic reticulum (RER) responsible for the segregation of secretory proteins within the intracisternal space of the RER. The first component of the RER to be purified and characterized is signal peptidase, the enzyme responsible for removing the signal peptide from nascent presecretory proteins. We have succeeded in maintaining the activity of the solubilized enzyme over long periods of time and we have recently reported that phosphatidylcholine is a required cofactor for the processing of presecretory proteins by solubilized signal peptidase (R.C. Jackson and W.R. White (1981) J. Biol. Chem. 256, 2545-2550). These advances have permitted us to obtain a substantial purification of signal peptidase. We are proceeding with the purification and characterization of this enzyme.